An X-ray generator is a device incorporating the X-ray tube for emitting X rays in it and often used for medical or industrial diagnostic devices, etc. The X-ray tubes are also practically used in various types according to the uses of the X-ray generators. For example, when an object to be inspected is inspected for its microstructure with X rays, the X-ray tube, the so-called micro focus X-ray tube, having a focal size of the electron beam of approximately several μm to dozens μm on an anode target which is a generating field of the X rays is used (e.g., Japanese Patent Laid-Open Application No. 2001-273860).
For example, the above-described micro focus X-ray tube has a structure in which an anode target for emitting X rays and a cathode are respectively disposed within a vacuum vessel. The cathode is comprised of a cathode electrode for generating an electron beam by heating by a heater, a grid electrode for controlling a tube current and a focus electrode for controlling a focal size of the electron beam on the anode target.
Generally, the X-ray tube having the above-described structure determines, for example, a cathode electrode, an anode target or a grid electrode to a ground potential and impresses a prescribed tube voltage on the anode target. An operational state of the X-ray tube is adjusted by, for example, controlling a voltage to be impressed on the focus electrode and the grid electrode. To control the voltage to be impressed on the focus electrode, a power supply for the focus electrode for generating a focus voltage to be impressed on the focus electrode is used independent of the power supply for the anode target for generating the tube voltage.
According to the focus voltage controlling method, however, when the tube voltage to be impressed on the anode target or the focus voltage to be impressed on the focus electrode has a change such as pulsation, a focal shape of the electron beam is affected, and it becomes difficult to form a micro focal point. Specifically, where the focal shape of the electron beam is minimized, it is significant to maintain the proportional relationship between the tube voltage and the focus voltage as indicated by, for example, code P in FIG. 7. If the tube voltage or the focus voltage changes, the proportional relationship shown in FIG. 7 is not maintained, and it becomes difficult to form the micro focal point. It was confirmed through the experiments made by the present inventors that a change in ratio between the tube voltage and the focus voltage by 0.15% has a large influence on the focal diameter.
On the other hand, for example, Japanese Patent Laid-Open Application No. Hei 7-29532 describes an X-ray generator which sets the focus electrode to a ground potential and changes the voltage to be impressed on the cathode electrode at a prescribed ratio in response to a change in voltage to be impressed on the anode target. According to the above existing X-ray generator, the focus electrode keeps the ground potential and does not change it, so that the micro focal point can be maintained stably even if the voltage impressed on the anode target suffers from pulsation.
However, the X-ray generator described in the above publication must have the focus electrode set to a ground potential, so that its device structure is highly limited. For example, the existing X-ray generator generally has the anode target or the grid electrode set to the ground potential, but the micro focal point forming method described in the above publication cannot be applied to the above X-ray generator. Therefore, to set the anode target or the grid electrode to the ground potential, there are demands for a technology which can suppress an effect of a variation in voltage on the formation of the micro focal point of the electron beam.
In the micro focus X-ray tube, the bias voltage is impressed between the cathode electrode and the grid electrode to control a current (tube current) of the electron beam for generating X rays by this bias voltage. Where this tube current control method is applied, it is common to separately dispose a power supply for generating the bias voltage.
However, the above tube current control method allows an excessively large tube current to pass through the X-ray tube if the power supply for the bias voltage fails. Such an excessively large tube current causes melting of the anode target, resulting in degradation of the properties of the X-ray tube and also its destruction and the like. Therefore, it is desired to improve the reliability and safety when the tube current is controlled by the bias voltage impressed on the cathode electrode.
It is an object of the present invention to provide an X-ray generator which can suppress an effect of a variation in voltage upon the formation of a focal point of the electron beam when the anode target or the grid electrode is set to a ground potential. Another object of the invention is to provide an X-ray generator having its reliability and safety improved by preventing the passage of an excessively large tube current when the tube current is controlled by the bias voltage to be impressed on the cathode electrode.